1. Field of the Invention
This invention relates to a microwave power amplifier/combiner for combining the output powers of a plurality of reflection type microwave amplifiers and, more particularly, to an improvement in a microwave power amplifier/combiner which employs a .pi./2 hybrid coupler.
2. Description of the Prior Art
A typical conventional power amplifier/combiner for combining the output powers of a plurality of reflection amplifiers has employed a .pi./2 phase difference 3-dB hydrid coupler (referred to as HYB). Before the conventional power amplifier/combiner employing the HYB is described, the HYB will first be described.
FIG. 1 shows an example of the HYB, and the function of the HYB 1 is as follows: When an electric power having an amplitude A.sub.1 and a phase P.sub.1 is supplied from the port 1-1 of the HYB, outputs whose amplitudes and phases respectively A.sub.2, P.sub.2 ; A.sub.3, P.sub.3 ; and A.sub.4, P.sub.4 are obtained at ports 1-2, 1-3 and 1-4. When the HYB is ideally constructed, the following relationship can be obtained: EQU A.sub.3 =A.sub.4 =(A.sub.1 /2) (1) EQU P.sub.4 -P.sub.3 =(.pi./2) (2)
When the ports 1-3 and 1-4 are matched, EQU A.sub.2 =0 (3)
In this case, the port 1-2 is referred to as an isolation port.
In constructing the HYB, a parallel line type directional coupler, a branch line type coupler, etc. are employed in a plane circuit type, and a side wall coupler, a multihole coupler, etc. are employed in a waveguide type.
FIG. 2 shows a typical conventional amplifier/combiner for combining the output powers of two reflection amplifiers using the HYB shown in FIG. 1. The reflection amplifier referred to here is an amplifier of a type which amplifies the electric power inputted from a port of the amplifier and outputs the output at the same port. In an actual construction, the reflection amplifier employs a microwave semiconductor device such as an IMPATT diode or Gunn diode. If an operating point is appropriately chosen, a negative resistance amplifier or an injection locking amplifier can be constructed.
In FIG. 2, when the electric power (A.sub.1, P.sub.1) is inputted from the port 1-1, output power (A.sub.3, P.sub.3) and (A.sub.4, P.sub.4) is outputted from the respective ports 1-3 and 1-4. It is assumed here that the power gains of the reflection amplifiers 2 and 3 are an equal value G, and an equal phase amount of P.sub.A is shifted.
The electric powers amplified by the amplifiers 2 and 3 are respectively outputted from the ports 1-1 and 1-2. The amplitude and phase of a microwave signal appearing at the port 1-1, which is amplified and reflected by the amplifier 2 and inputted from the port 1-3 into the HYB 1, is represented by A.sub.1-3 and P.sub.1-3, and the amplitude and phase of the microwave at the port 1-2 is represented by A.sub.2-3 and P.sub.2-3. Similarly, the amplitudes and phases of the microwave signal appearing at the ports 1-1 and 1-2, which are amplified and reflected by the amplifier 3, and inputted from the port 1-4 into the HYB 1, are respectively represented by A.sub.1-4, P.sub.1-4, A.sub.2-4, P.sub.2-4. For the simplicity of description, the phase delays of the transmission line except the coupler port of the HYB are assumed to be equal for 4 branch lines and therefore can be neglected, and the insertion loss caused by the ohmic loss of the HYB is ignored P.sub.1 =P.sub.3 and P.sub.4 =P.sub.1 +.pi./2 are also assumed.
From the above-described assumption, the following equations can be obtained: EQU A.sub.1-3 =A.sub.1-4 =A.sub.2-3 =A.sub.2-4 ( 4) EQU P.sub.1-3 =P.sub.3 +P.sub.A ( 5) EQU P.sub.1-4 =P.sub.4 +.pi./2 (6) EQU P.sub.2-3 =P.sub.3 +P.sub.A +.pi./2 (7) EQU P.sub.2-4 =P.sub.4 +P.sub.A ( 8)
Since the circuit is considered to have no loss, A.sub.1-3 +A.sub.1-4 +A.sub.2-3 +A.sub.2-4 =GA.sub.1 can be obtained. However, since P.sub.4 =P.sub.3 +.pi./2, the equations (6) and (8) can be modified as follows: EQU P.sub.1-4 =P.sub.1 +P.sub.A +.pi. (9) EQU P.sub.2-4 =P.sub.3 +P.sub.A +.pi./2 (10)
From the equations (5) and (9) the microwaves being equal in amplitude and opposite in phase are produced at the port 1-1, which are cancelled with each other. Further, from the equation (10) the microwaves being equal in amplitude and same in phase are produced at the port 1-2 which are added to one another. In other words, the electric power, respectively amplified and reflected by the amplifiers 2 and 3, is not propagated to the port 1-1, but is outputted as the combined output of A.sub.2-3 +A.sub.2-4 =GA.sub.1 to the port 1-2. In this manner, this circuit functions, although using the reflection amplifiers, as amplifier/combiner of a passing type amplifier having different input and output ports and does not necessitate a circulator which divides into input and output power ports as required in the reflection type amplifier.
FIG. 3 shows a typical conventional 4-amplifier/combiner for combining the outputs of four reflection type amplifiers.
This circuit has four HYBs 4 to 7 and four reflection amplifiers 8 to 11. Loads 12 and 13 are dummy loads. The HYBs 4 to 7 respectively have ports 4-1 to 4-4, 5-1 to 5-4, 6-1 to 6-4 and 7-1 to 7-4. The electric power inputted from the port 4-1 having an amplitude of 1/2 and a phase difference of /2 is distributed to the ports 4-3 and 4-4.
The output electric powers of the ports 4-3 and 4-4 are, as described with respect to FIG. 2, respectively amplified through two amplifiers/combiners of 2-amplifier/combiner having HYB 5 and amplifiers 8 and 9, and 2-amplifier/combiner having HYB 7 and amplifiers 10 and 11, and are fed to the ports 6-1 and 6-2. Since the microwave thus amplified is maintained with the phase difference .pi./2 produced at the HYB 4, it becomes in opposite phase at the port 6-3 and becomes in phase at the port 6-4. In other words, the electric power inputted from the port 4-1 is amplified through the amplifiers 8 to 11, and the combined output is outputted to the port 6-4. Such a 4-amplifier amplifier/combiner is disclosed in IEE Transactions On Microwave Theory And Techniques, Vol. MTT-24, No. 11, November 1976, P.766, FIG. 9(a).
Similarly, a typical conventional 8-amplifier amplifier/combiner is shown in FIG. 4.
In this manner, 2.sup.n pieces (n=1, 2, 3, . . . ) of amplifiers can be coupled through the HYB, and in the conventional amplifier/combiner, (3.times.2.sup.n-1 -2) pieces of HYBs should be employed to combine the outputs of 2.sup.n pieces of amplifiers.